Driving circuit and method for liquid crystal display panel

ABSTRACT

A driving circuit and a driving method can drive an LCD panel to display imagines. Storages capacitors of pixels connected with each scanning line are connected with an AC signal source. The AC signal source can vary the potential of its signal in harmony with the polarity inversion of a pixel during a vertical scanning period. Due to a capacitively coupled effect, a coupled voltage induced by the variation in the potential of the signal changes the potential of a pixel electrode so as to speed up the alternation in the electrical field of an LC capacitor.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a driving circuit and a drivingmethod for a liquid crystal display (LCD) panel, and more particularlyto a driving circuit and a driving method for an active matrix LCD panelcapable of shortening response time.

[0003] 2. Description of the Related Art

[0004] The LCD technology has progressed in the manufacture of highcontrast and wide view angle flat displays. However, for the dynamicimage that displays a continuous movement, the image qualitydeteriorates due to blur images caused by a response delay. Recently,there have been many relative driving methods to improve the responsetime of LCD panels, and the capacitively coupled driving (CCD) methodprovided by Matsushita Electric Industrial Co., Ltd. is one superiorsolution which has a fast response to charge the potentials of pixelelectrodes. Therefore, the electrical field of an LC capacitor changesvery fast after a gradation voltage being written therein.

[0005]FIG. 1 is an equivalent circuit diagram of a conventional LCDpanel. The LCD panel 10 has a plurality of pixels 13 formed by aplurality of data lines 121-12 n crossing a plurality of scanning lines111-11 m. Each of the pixels 13 includes a thin film transistor (TFT)131 and an LC capacitor 133 that controls the rotation directions of LCmolecules. A TFT 131 can be turned on and off by the scanning signal Φ₁applied to the scanning line 112. The two terminals of the LC capacitor133 are separately connected with a pixel electrode 134 and a commonelectrode 135. Furthermore, a storage capacitor 132 included in thepixel 13 also has two terminals respectively connected with the pixelelectrode 134 and the scanning line 111. The existence of the storagecapacitor 132 can keep the potential of the pixel electrode 134 in anadequate variable range, and reduces current leakage resulted from theproperties of LC materials and undesired parasitic capacitors.

[0006]FIG. 2 is a waveform diagram of the potentials of scanning signalsand a pixel electrode applied by a conventional CCD method. During avertical scanning period (or a frame time), scan signals Φ₁, Φ₂, . . . ,Φ_(m) are respectively applied to scanning lines 111-11 m in sequence,and each scanning signal can sequentially turn on the TFTs 131 connectedwith the corresponding scanning line so as to allow a corresponding datasignal to be written into the LC capacitor 133. The CCD method enablestwo adjacent scanning lines to be respectively input signals Φ_(k−1) andΦ_(k) of four potential levels V1-V4. Because the storage capacitor 132is connected with a previous scanning line, a coupled voltage Vcc isapplied to the pixel electrode 134 from the variation of the potentialsof the signal Φ_(k−1) so that V_(p)=Vs+Vcc, wherein Vp represents thecurrent potential of the pixel electrode 134, Vs represents thegradation voltage supplied by the data line and Vcc represents thecoupled voltage applied to the pixel electrode 134.

[0007] FIGS. 3(a)-4(b) show waveform diagrams of overshooting orundershooting variations in the potentials of a pixel electrode 134. Dueto these abrupt changes, the LC capacitor 133 can rapidly adjust itselectrical field to a predefined one. As shown in FIGS. 3(a)-3(b), thepotentials of the pixel electrode 134 vary when the electrical field ofthe LC capacitor 133 changes from low to high. Referring to FIG. 3(a),the potential of the pixel electrode abruptly rises due to a coupledvoltage when the pixel is defined from a negative polarity to a positivepolarity. The rise of the potential Vcc(+) is the magnitude of thecoupled voltage and is regarded as an overshooting phenomenon. Furtherreferring to FIG. 3(b), the potential of the abruptly falls due to acoupled voltage when the pixel is defined from a positive polarity to anegative polarity. The fall of the potential Vcc(−) is the magnitude ofthe coupled voltage and is regarded as an overshooting phenomenon.

[0008] Furthermore, as shown in FIGS. 4(a)-4(b), the potentials of thepixel electrode 134 vary when the electrical field of the LC capacitor133 changes from high to low. Referring to FIG. 4(a), the potential ofthe abruptly rises due to a coupled voltage when the pixel is definedfrom a negative polarity to a positive polarity. The rise of thepotential Vcc(+) is the magnitude of the coupled voltage and is regardedas an undershooting phenomenon. Further referring to FIG. 4(b), thepotential of the abruptly falls due to a coupled voltage when the pixelis defined from a positive polarity to a negative polarity. The fall ofthe potential Vcc(−) is the magnitude of the coupled voltage and isregarded as an undershooting phenomenon.

[0009] The CCD method, a prior art technology/technique, is also calleda four potential levels driving (including four potential levels V1-V4),wherein V1 and V3 can respectively turn on and off the TFT 131, and V2and V4 are driving potentials to induce coupled voltages Vcc. Theproperties of the TFT 131 determine the magnitude of V1 and V3. Inaddition, the magnitude of V2 and V4 limited by V1 and V3 has a narrowadjustable range, so that the magnitude of the coupled voltage is undera specific value. On the other hand, specific driving devices are neededfor generating the scanning signal consisting of four potential levels.Therefore, it is difficult to obtain these driving devices for apractical application. Furthermore, the RC delay on a scanning linebecomes worse due to the connection between the storage capacitors 132and the scanning line.

[0010] Because the conventional driving circuit of an LCD panel isunable to independently control the magnitude of the coupled voltage,data driving devices still need to output the data signals with widepotential ranges so that it is hard to meet the requirement of the LCDmarket.

SUMMARY OF THE INVENTION

[0011] An objective of the present invention is to provide a drivingcircuit and a driving method for an LCD panel whose storages capacitorsof pixels connected with each scanning line are connected with an ACsignal source, so coupled voltages applied to pixel electrodes can bemodulated line-byline or cluster-by-cluster. In comparison with theprior art that modulates the potential of a whole common electrode, thepresent invention substantially reduces power consumption and lowers themodulation frequency the prior art requires.

[0012] The second objective of the present invention is to provide aneasy applied method of capacitively coupled driving. It displayssuperior dynamic images to the four potential levels driving withoutemploying specially scanning driving devices.

[0013] The third objective of the present invention is to provide adriving circuit and a driving method for an LCD panel. It is compatiblefor various types of LCD panels including IPS (In-Plane Switching) typeand MVA (Multi-Domain Vertical Alignment) type.

[0014] The fourth objective of the present invention is to provide adriving circuit and a driving method for independently controlling theoccurrence of coupled voltages. The magnitude of the coupled voltage canbe larger than the magnitude of that resulted from the conventional CCDmethod, so that the range of voltages output by the data-driving devicecan be reduced.

[0015] In order to achieve the objective, the present inventiondiscloses a driving circuit and a driving method for an LCD panel.Storages capacitors of pixels connected with each scanning line areconnected with an AC signal source. The AC signal source can vary thepotential of its signal in harmony with the polarity inversion of apixel during a vertical scanning period. Due to a capacitively coupledeffect, a coupled voltage induced by the variation in the potential ofthe signal changes the potential of a pixel electrode so as to speed upthe alternation in the electrical field of an LC capacitor. Therefore,the LCD panel is suitable for displaying a fast continuous movement andreduces power consumption.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The invention will be described according to the appendeddrawings in which:

[0017]FIG. 1 is a circuit diagram in accordance with the LCD panel of aprior art reference;

[0018]FIG. 2 is a waveform diagram of the scanning signals and thepotential of a pixel electrode driven by a conventional capacitivelycoupled driving method;

[0019] FIGS. 3(a)-3(b) are waveform diagrams of the potentials of apixel electrode taken when the electrical field of an LC capacitorchanges from low to high;

[0020] FIGS. 4(a)-4(b) are waveform diagrams of the potentials of apixel electrode taken when the electrical field of an LC capacitorchanges from high to low;

[0021]FIG. 5 is an equivalent circuit diagram of a pixel in accordancewith the LCD panel of the present invention;

[0022] FIGS. 6(a)-6(b) are waveform diagrams of coupled voltages inducedby modulation signals in accordance with the first embodiment of thepresent invention; and

[0023] FIGS. 7(a)-7(b) are waveform diagrams of coupled voltages inducedby modulation signals in accordance with the second embodiment of thepresent invention.

PREFERRED EMBODIMENT OF THE PRESENT INVENTION

[0024]FIG. 5 is an equivalent circuit diagram of a pixel in accordancewith the LCD panel of the present invention. The pixel 50 can beregarded as any one of the pixels of an LCD panel, that is, all pixelshave the same circuit layout. Two parallel scanning lines 512 and 513are respectively perpendicular to two parallel data lines 524 and 525,wherein an enclosed area is the region of the pixel 50. The gateterminal and source terminal of a thin film transistor (TFT) 53 arerespectively connected with the scanning line 513 and the data line 524.After the TFT 53 is selected to be turned on by a scanning signalapplied to the scanning line 513, a data signal is written into an LCcapacitor 56 whose another terminal is connected with a common electrode55. Furthermore, the pixel 50 comprises a first storage capacitor 541, asecond storage capacitor 542 and a third storage capacitor 543 capableof stabilizing the voltage applied to the LC capacitor 56, so that thedrop of the voltage caused by a feed-through effect can be reduced. Thesecond storage capacitor 542 and third storage capacitor 543 can beexcluded in some cases, that is, they are optional devices for thepresent invention.

[0025] Another terminal of the second storage capacitor 542 is connectedwith common electrode 55; another terminal of the third storagecapacitor 543 is connected with a previous scanning line 512; andanother terminal of the first storage capacitor 541 is connected with amodulation signal source 57. The potential of the pixel electrode 59 canbe modulated by a modulation signal from the modulation signal source 57and a capacitively coupled effect, so that the electrical field of theLC capacitor 56 is fast driven to vary therein. The first storagecapacitors 541 of pixels connected with a same scanning line 513 can beconnected with a signal source 57. Therefore, the potentials of thepixel electrodes 59 are modulated line-byline and pixel-by-pixel inaccordance with the scanning sequence of scanning lines. Due to thelimitation of the manufacturing process, a parasitic capacitor 59certainly exists between the gate terminal and drain terminal of the TFT53, and results in the feedthrough effect.

[0026] FIGS. 6(a)-6(b) are waveform diagrams of coupled voltages inducedby modulation signals in accordance with the first embodiment of thepresent invention. The modulation signal is applied to a pixel prior tothat a scanning signal turns on the TFT. FIG. 6(a) represents that thepolarity of the pixel changes from negative to positive. Because asquare pulse rising from a lower level to a higher level, acting as themodulation signal, is written into the first storage capacitor 541, thepotential of the pixel electrode 59 rises from an initial level to afirst level in advance by a capacitively coupled effect. The rise Vcc(+) of the potential is a capacitively coupled voltage. After that, theTFT 53 is turned on by a scanning signal. A data signal from data line524 is written into the pixel electrode 59 to change the potential ofthe pixel electrode 59 from the first level to the second level, and thevariation of the potential of the pixel electrode 59 is equal to thepotential of the data signal. FIG. 6(b) represents that the polarity ofthe pixel changes from positive to negative. Similarly, a square pulse,falling from a higher level to a lower level, acting as the modulationsignal is written into the first storage capacitor 541. Therefore, thepotential of the pixel electrode 59 abruptly falls from a initial levelto a first level in advance of the change of the polarity by thecapacitively coupled effect, and the fall Vcc (−) of the potential is acapacitively coupled voltage. After that, the TFT 53 is turned on by ascanning signal. A data signal from data line 524 is written into thepixel electrode 59 to change the potential of the pixel electrode 59from the first level to the second level, and the variation of thepotential of the pixel electrode is equal to the potential of the datasignal. As shown FIG. 6(a), in order to have a better image quality inthe preferred embodiment of the present invention, the higher level ofthe potential of the modulation signal is higher than the second levelof the potential of the pixel electrode 59 when the polarity of thepixel changes from negative to positive. On the other hand, as shown inFIG. 6(b), the lower level of the potential of the modulation signal islower than the second level of the potential of the pixel electrode 59when the polarity of the pixel changes from positive to negative.

[0027] FIGS. 7(a)-7(b) are waveform diagrams of coupled voltages inducedby modulation signals in accordance with the second embodiment of thepresent invention. The modulation signal is applied to a pixel posteriorto that a scanning signal turns off the TFT. FIG. 7(a) represents thatthe polarity of the pixel changes from negative to positive. Whenturning on the TFT 53 by a scanning signal, the potential of the pixelelectrode 59 rises from an initial level to a first level because a datasignal from the data line 524 is written into the pixel electrode 59.The difference between the initial level and the first level of thepixel electrode 59 potential is equal to the potential of the datasignal. After that, a square pulse rising from a lower level to a higherlevel, acting as the modulation signal, is written into the firststorage capacitor 541. The potential of the pixel electrode 59 furtherrises from the first level to the second level by a capacitively coupledeffect. The rise Vcc (+) of the potential is a capacitively coupledvoltage. FIG. 7(b) represents that the polarity of the pixel changesfrom positive to negative. Similarly, when turning on the TFT 53 by ascanning signal, the potential of the pixel electrode 59 falls from aninitial level to a first level because a data signal from the data line524 is written into the pixel electrode 59. The difference between theinitial level and the first level of the potential of the pixelelectrode 59 is equal to the potential of the data signal. After that, asquare pulse falling from a higher level to a lower level, acting as themodulation signal, is written into the first storage capacitor 541. Thepotential of the pixel electrode 59 further falls from the first levelto the second level by a capacitively coupled effect. The fall Vcc (−)of the potential is a capacitively coupled voltage. As shown FIG. 7(a),in order to have a better image quality in the preferred embodiment ofthe present invention, the higher level of the potential of themodulation signal is higher than the second level of the potential ofthe pixel electrode when the polarity of the pixel changes from negativeto positive. On the other hand, as shown in FIG. 7(b), the lower levelof the potential of the modulation signal is lower than the second levelof the potential of the pixel electrode when the polarity of the pixelchanges from positive to negative.

[0028] Since the present invention discloses that a modulation signal iswritten into the first storage capacitor from an isolated modulationsignal source and then a coupled voltage is induced on the pixelelectrode, it has many degrees of freedom to design the waveform shapeof the modulation signal. In comparison with the prior art disclosing ascanning signal with four potential levels, this method can have acoupled voltage with higher magnitude induced by the isolated modulationsignal source. Therefore, the maximum amplitude of the data signal canbe reduced and the power consumption of a data-driving device can besaved.

[0029] The above-described embodiments of the present invention areintended to be illustrative only. Numerous alternative embodiments maybe devised by persons skilled in the art without departing from thescope of the following claims.

What is claimed is:
 1. A driving circuit for a liquid crystal displaypanel, comprising: a plurality of data lines; a plurality of scanninglines; a common electrode; a plurality of pixels positioned onintersections of the scanning lines and the data lines, each of theplurality of pixels including: a thin film transistor whose gateelectrode, source electrode and drain electrode are separately connectedto the scanning line, the data line and a pixel electrode; a liquidcrystal capacitor whose two terminals are separately connected to thepixel electrode and the common electrode; and a first storage capacitorhaving one terminal electrically connected to the pixel electrode andhaving another terminal electrically connected to a modulation signalsource providing modulation signals to the first storage capacitors ofthe pixels so as to generate corresponding coupled voltages.
 2. Thedriving circuit for a liquid crystal display panel of claim 1, whereineach of the plurality of pixels further comprises a second storagecapacitor whose two terminals are separately connected to the pixelelectrode and the common electrode.
 3. The driving circuit for a liquidcrystal display panel of claim 1, wherein each of the plurality ofpixels further comprises a third storage capacitor whose two terminalsare separately connected to the pixel electrode and the scanning lineadjacent to the pixel, and the scanning line adjacent to the pixel iselectrically isolated from the gate terminal of the thin film transistorof the pixel.
 4. The driving circuit for a liquid crystal display panelof claim 1, wherein the first storage capacitors positioned on the samescanning line are electrically connected to a same modulation signalsource.
 5. The driving circuit for a liquid crystal display panel ofclaim 1, wherein the modulation signal source generates a square pulseas the modulation signal.
 6. A driving method for a liquid crystaldisplay panel that includes a plurality of matrix-arranged pixelspositioned on intersections of a plurality of scanning line and aplurality of data lines, each pixels having a thin film transistor whosegate electrode, source electrode and drain electrode are separatelyconnected to the scanning line, the data line and a pixel electrode anda first storage capacitor whose one terminal electrically connected tothe pixel electrode and another terminal electrically connected to amodulation signal source, comprising the steps of: starting a scanningperiod for the scanning line; writing sequentially a modulation signalprovided by the modulation signal source connected to the pixelspositioned on the scanning line into each of the first storagecapacitors; inducing a coupled voltage to change the potential of thepixel electrode from an initial level to a first level through themodulation signal applied to the first storage capacitor, wherein thevariation of the potential of the pixel electrode is equal to thecoupled voltage; turning on sequentially the thin film transistors ofthe pixels on the scanning line by a scanning signal; writing a datasignal from the data line into the pixel electrode to change thepotential of the pixel electrode from the first level to a second level,wherein the variation of the potential of the pixel electrode is equalto the potential of the data signal; and ending the scanning period forthe scanning line.
 7. The driving method for a liquid crystal displaypanel of claim 6, wherein the potential of the modulation signal changesfrom a lower level to a higher level as square pulse, and meantime thepolarity of the pixel changes from negative to positive during thescanning period.
 8. The driving method for a liquid crystal displaypanel of claim 7, wherein the higher level of the potential of themodulation signal is higher than the second level of the potential ofthe pixel electrode.
 9. The driving method for a liquid crystal displaypanel of claim 6, wherein the higher level of the potential of themodulation signal changes from a higher level to a lower level as asquare pulse, and meantime the polarity of the pixel changes frompositive to negative during the scanning period.
 10. The driving methodfor a liquid crystal display panel of claim 9, wherein the lower levelof the potential of the modulation signal is lower than the second levelof the potential of the pixel electrode.
 11. The driving method for aliquid crystal display panel of claim 6, wherein the pixel furthercomprises a second storage capacitor whose two terminals are separatelyconnected to the pixel electrode and a common electrode.
 12. The drivingmethod for a liquid crystal display panel of claim 6, wherein the pixelfurther comprises a third storage capacitor whose two terminals areseparately connected to the pixel electrode and the scanning lineadjacent to the pixel, and the scanning line adjacent to the pixel iselectrically isolated from the gate terminal of the thin film transistorof the pixel.
 13. The driving method for a liquid crystal display panelof claim 6, wherein the first storage capacitors of all the pixels onthe scanning line are all connected to the signal modulation source. 14.A driving method for a liquid crystal display panel that includes aplurality of matrix-arranged pixels positioned on intersections of aplurality of scanning line and a plurality of data lines, each pixelshaving a thin film transistor whose gate electrode, source electrode anddrain electrode are separately connected to the scanning line, the dataline and a pixel electrode and a first storage capacitor whose oneterminal electrically connected to the pixel electrode and anotherterminal electrically connected to a modulation signal source,comprising the steps of: starting a scanning period for the scanningline; turning on sequentially the thin film transistors of the pixels onthe scanning line by a scanning signal; writing a data signal from thedata line into the pixel electrode to change the potential of the pixelelectrode from an initial level to a first level, wherein the variationof the potential of the pixel electrode is equal to the potential of thedata signal; writing sequentially a modulation signal provided by themodulation signal source connected to the pixels positioned on thescanning line into each of the first storage capacitors; inducing acoupled voltage to change the potential of the pixel electrode from thefirst level to a second level through the modulation signal applied tothe first storage capacitor, wherein the variation of the potential ofthe pixel electrode is equal to the coupled voltage; and ending thescanning period for the scanning line.
 15. The driving method for aliquid crystal display panel of claim 14, wherein the potential of themodulation signal changes from a lower level to a higher level as squarepulse, and meantime the polarity of the pixel changes from negative topositive during the scanning period.
 16. The driving method for a liquidcrystal display panel of claim 15, wherein the higher level of thepotential of the modulation signal is higher than the second level ofthe potential of the pixel electrode.
 17. The driving method for aliquid crystal display panel of claim 14, wherein the potential of themodulation signal changes from a higher level to a lower level as asquare pulse, and meantime the polarity of the pixel changes frompositive to negative during the scanning period.
 18. The driving methodfor a liquid crystal display panel of claim 17, wherein the lower levelof the potential of the modulation signal is lower than the second levelof the potential of the pixel electrode.
 19. The driving method for aliquid crystal display panel of claim 14, wherein the pixel furthercomprises a second storage capacitor whose two terminals are separatelyconnected to the pixel electrode and a common electrode.
 20. The drivingmethod for a liquid crystal display panel of claim 14, wherein the pixelfurther comprises a third storage capacitor whose two terminals areseparately connected to the pixel electrode and the scanning lineadjacent to the pixel, and the scanning line adjacent to the pixel iselectrically isolated from the gate terminal of the thin film transistorof the pixel.
 21. The driving method for a liquid crystal display panelof claim 14, wherein the first storage capacitors of all the pixels onthe scanning line are all connected to the signal modulation source.